Roles of endogenous opioid peptides and their receptors in modulation of the nocifensive responses to formalin in mice were studied. Mice were pretreated i.c.v. or intrathecally (i.t.) with selective opioid receptor antagonists or intrathecally with antisera against endogenous opioid peptides and the nocifensive licking responses to intraplantar injection of formalin (0.5%, 25 microl) were then observed. Pretreatment with the epsilon-opioid receptor antagonist beta-endorphin(1-27) or the selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP) given i.c.v. dose dependently enhanced the second, but not the first phase of the nocifensive response. However, i.c.v. pretreatment with the selective delta-receptor antagonist naltrindole or kappa-receptor antagonist nor-binaltrophimine did not affect the nocifensive responses. Intrathecal pretreatment with selective delta(1)-opioid antagonist 7-benzylidene naltrexamine significantly enhanced both the first and second phases of nocifension. Intrathecal pretreatment with CTOP also increased the second but not the first phase of the nocifension. However, i.t. pretreatment with the selective delta(2)-receptor antagonist naltriben or nor-binaltrophimine did not affect the second phase of the nocifension. Intrathecal pretreatment with antiserum against Leu-enkephalin, Met-enkephalin, or dynorphin A(1-17), but not beta-endorphin, enhanced only the second phase of nocifensive response to formalin. It is concluded that the blockade of epsilon- and mu-receptors, but not delta- or kappa-receptors, at the supraspinal sites enhanced the second phase of formalin-induced nocifension. In the spinal cord, Leu-enkephalin, and to a lesser extent, Met-enkephalin and dynorphin A(1-17) and mu- and delta(1)-opioid receptors, but not delta(2)- or kappa-opioid receptors, are involved in modulating the feedback inhibition of the second phase of formalin-induced nocifension.